The invention relates to apparatus and a method for servicing a nuclear reactor system wherein the cold leg entrance nozzles of a nuclear reactor vessel are plugged so that servicing and inspection of the reactor and its associated components may be carried out simultaneously.
In the conventional pressurized water nuclear reactor system the nuclear reactor is disposed within an annulus of a containment structure located in the reactor building. The containment annulus and reactor vessel are disposed below a refueling canal which is filled with water during refueling. A refueling bridge is disposed above the canal which assists in the refueling and inspection operations.
In a typical nuclear reactor installation, the nuclear reactor vessel must be routinely inspected. This means the reactor must be shut down and the fuel and core support assemblies removed. During this inspection, the fuel and core support assemblies must be removed from the nuclear reactor vessel and the interior of the vessel inspected for cracks in weld joints, and other possible sources of leakage. In order to remove the fuel and core support assemblies, the water canal above the reactor must be filled with water in order to provide shielding for the fuel assemblies and the core support assemblies since they are radiologically very hot. The core support assembly is placed in a deep end of the canal and the fuel assemblies are maintained in the spent fuel pool, both of which must be maintained under water at all times for shielding. Since the top closure is off of the reactor vessel, the vessel is filled with water. The interior of the nuclear reactor vessel is inspected by means of remote control and automatic inspection equipment that is lowered into the interior of the vessel.
Generally, there are two types of steam generators used in pressurized water reactors. The U-tube steam generator is typically utilized in systems such as that manufactured by The Westinghouse Company and Combustion Engineering, Inc. These steam generators have an entrance port in the lower portion of the steam generator unit through which water heated by the nuclear reactor passes and travels upwardly through the steam generator and back down through a U-tube. The water then exits the steam generator at about the same level at which it enters. The other type of steam generator unit such as that manufactured by the Babcock & Wilcox Co. is commonly referred to as the once-through type steam generator unit. In this type of steam generator unit the water heated by the nuclear reactor travels upwardly and enters the top of the steam generator unit whereupon it travels down through the unit and leaves the unit at the lower portion of the unit. A pump unit pumps the water back into the nuclear reactor through an entrance nozzle at an elevated position which corresponds with the level of the outlet nozzle for heated water.
It would be desirable to be able to inspect the associated steam generator and pump units at the same time that the interior of the reactor vessel is being inspected. However, water is present in the canal and in the interior of the reactor vessel, and because of the elevated position of the canal water level relative to the pump unit and steam generator, water flows through the cold leg piping system connecting the reactor vessel to the steam generator and pump units. The pump unit and steam generator units are flooded. Servicing of the steam generator and pump units is not possible until after the reactor vessel is refueled and re-assembled and the water drained therefrom and from the canal.
Typically, it takes approximately three weeks to inspect the vessel and refurbish the vessel and drain the water fom the canal before the pumps and steam generator units may be inspected prior to the reactor being reactivated. If the steam generator units and pumps could be serviced simultaneously with the nuclear vessel, a considerable amount of savings in time and money could be had along with a quicker return of the unit to service all of which would result in quicker resumption of the generation of electrical and power and income therefrom.
Heretofore, it has been attempted to place expandable bladders into the entrance nozzles of the cold leg piping leading from the reactor vessel to the steam generator. However, these bladders have presented considerable difficulty in staying in place and have become dislodged resulting in water flowing into the pumps and steam generator units during servicing. Since the water is radioactive, this would present a harzardous condition to the servicemen.
It has been proposed in a paper published by The American Society of Mechanical Engineers entitled "Steam Generator Nozzle Dams for Primary Head Activities", publication no. 80-C2/NE-11 by Lewis and Woods to plug the nozzles leading from A U-type steam generator unit to the reactor for the purpose of inspecting the steam generator tubing. This is done by plugging the nozzles in the steam generator unit. However, this does not provide access for servicing the pump units which would remain flooded even if the steam generator nozzles were plugged, which would be particularly problematic in once-through type steam generators. The sealing plugs proposed in this paper are manually inserted by hand.
It has also been proposed in boiling water reactors such as those manufactured by The General Electric Company, that the steam line leading from the reactor vessel to the turbines be plugged from the interior of the reactor vessel. However, this type plug requires that there be no water present in the water canal in order for it to be utilized. Moreover, this plug and the associated apparatus for inserting the plug into the nozzle must remain in place during use. This would render such apparatus unusable for servicing a pressurized water nuclear reactor vessel which must be filled with water and completely open from the top for entry of automatic inspection equipment.
Numerous methods and devices for plugging pipelines have been proposed, for example, in U.S. Pat. Nos. 3,381,714; 3,483,895; and 3,495,626, however, none of the above presents a suitable device or method for servicing a nuclear reactor system which requires a high degree of reliability in sealing off the system and presents different problems due to the radioactive environment and other associated problems such as submergence of the reactor vessel.
Accordingly, an important object of the present invention is to provide a method and apparatus which will permit servicing of a pressurized nuclear reactor vessel and its associated steam generator and pump units during the time that the refueling canal is filled with water.
Still another important object of the present invention is to provide a method and apparatus for plugging the entrance nozzles of a cold leg piping system of a pressurized water nuclear reactor vessel to permit servicing and inspection of the vessel and associated steam generator and pump units at the same time during refueling of the reactor.
Still another important object of the present invention is to provide a method and apparatus for plugging the nozzles of a nuclear reactor system during refueling so that the steam generator and pump units may be serviced simultaneously with the reactor vessel.
Still another important object of the present invention is to provide a method and apparatus for installing sealing plugs in the nozzles of a pressurized water nuclear reactor vessel which is submerged in water from a remote location.
Still another important object of the present invention is to provide a method and apparatus for installing sealing plugs in the entrance nozzles of a pressurized water reactor vessel wherein the sealing plugs of opposed nozzles are inserted simultaneously from opposite directions whereafter the installation mechanism may be removed to accommodate the utilization of automatic inspection equipment within the vessel.
Still another important object of the present invention is to provide a method and apparatus for installing sealing plugs in the entrance nozzles of a pressurized water reactor vessel which automatically aligns the sealing plugs for installation and provides a reactionary force against which the plug may be removed in the case of difficulty.
Yet another important object of the present invention is to provide a sealing plug for an entrance nozzle of a pressurized water reactor vessel which has a primary sealing means for sealing against a wall of an entrance nozzle and a secondary sealing means which provides a backup seal and prevents the seal from being dislodged.
Still another important object of the present invention is to provide a sealing plug for an entrance nozzle of a pressurized water reactor vessel having a contoured portion by which the plug is retained in the nozzle under hydrostatic pressure.